We propose to study structure of insect flight muscle in the rigor state by the method of 3-dimensional image reconstruction from electron micrographs. The goal of this research is a detailed description of the rigor state including an explanation of the various types of images obtained. These include the "flared X" and double chevron. We hope to determine independently the geometry of crossbridge binding to actin, test the models for 2 filament myosin interactions and obrain an understanding of the limits of movement placed on the crossbridge during actin binding. We propose to study by electron microscopy, partially decorated actin filaments and actin filaments polymerized onto decorated actin using rapid freezing techniques to test the geometry of crossbridge binding to actin. We propose to study f-actin decorated with S1 heavy chain and with a modified S1 containing a heavy atom label on the alkali light chain, to determine the position of the label and to determine the position of the alkali light chain, using 3-dimensional image reconstruction methods. We propose to decorate "fixed" relaxed thin filaments with S1 to determine by 3-dimensional methods the position of tropomyosin. Using rapid-freezing, deep etching and rotary shadowing methods we propose to study the structure of actin filaments decorated with S1 in the presence of nucleotides to determine if there is any difference in their conformation relative to that in rigor. By the method of cryoultramicrotomy combined with computer image processing, we propose to study the axial distribution of matter in the vertebrate striated muscle A band and develop an explanation for the complex and detaled meridional x-ray diffraction pattern obained from it.